1. Searching for scintillation detector drift through analyses of recoil spectra from neutrons
scattered from 12
C and γ-rays emitted from radioactive sources 137
Cs, 60
Co, and 241
Am
S. G. Block, S. F. Hicks, M. T. Nickel, S. T. Byrd — University of Dallas
J. R. Vanhoy — United States Naval Academy
E. E. Peters, A. P. D. Ramirez, S. Mukhopadhyay, S. W. Yates — University of Kentucky
The program’s results [Examples are shown in Figs. 3 and 4.] indicate
that shifts in detector bias occur during certain periods of time. However,
these deviations do not heavily influence the behavior of the detector.
This can be seen in Fig. 3 where the shifts in nominal energy in the
backup files fall well within the range of error. The first two values can
be ignored due
to low statistics
at the
beginning of a
run. In Fig. 4
there is a small
inflection in the
energy versus
channel around
channels 2300
to 2500 which
distorts the
smooth
relationship.
The causes of
this deviation
are unknown,
but they do not appear in the date per channel analysis of the pulse-
height spectra and occur at every point of the experiment. Another way
to see this abnormality is by comparing the nominal energy channel with
the detector angle. One would expect three parallel curves to correspond
to the three beam energies. [See Figure 5.] However, one sees a point
around 80 degrees where the nominal energies converge, this
convergence corresponds to the inflection seen in Figure 4. Further work
will need to be done to find the cause of this behavior.
Figure 4: Nominal energy per channel analysis of 12C
Results
Figure 3 Nominal energy channel per timestamp of file. The dark gray box represents one σ,
the light gray box represents two σ, while the dotted line represents the weighted average.
Counting statistics become better from left to right, so initial deviations are not cause for
concern.
Figure 6: The experimental efficiency and control data from the experiment. the
smooth curves indicate no shifts in detector bias
References
Conclusions
In summary, changes in the detector bias do occur, but if the
measurement periods are small enough like the periods needed for
efficiency runs, these changes do not impact the experiment.
However, longer periods of data collection could be affected by these
changes. Further work will need to be done to find the sources of
these changes. Possible improvements to the program include
utilizing methods to eliminate background radiation from the
modeling process as well as subtracting previous counts from each
backup file in order to see the data collected during each backup
period. This would allow researchers to see the fluctuations caused
by changes in detector bias more clearly. Steps forward in this
experiment would include running this program on an experiment
where the power source was directly monitored to see direct
correlations between changes in the power source voltage and the
behavior of the experimental data. This could possibly show a
relation between certain wave distortions and the behavior of the
detector. This would inevitably lead to a more direct approach to
eliminating shifts in the detector bias.
- R. E, Philosophical Magazine 21, 669 (1911).
- P. Rinard, Passive Nondestructive Assay of Nuclear Materials, 357 (1991).
- T. Crane and M. Baker, Passive Nondestructive Assay of Nuclear Materials, 379 (1991).
- Glenn F. Knoll, Radiation Detection and Measurement, edited by B. Zorbist, R. Factor and S. Malinowski
(John Wiley & Sons, Hoboken, NJ, 2000), p. 802.
- V. Bildstein, P. E. Garrett, J. Wong, et al, Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment 729, 188 (2013).
- H. Navirian, Master's thesis, Lund University (2005).
Abstract
Detectors and Recoil Spectra
The University of Kentucky Accelerator Laboratory [UKAL] is
known for fast neutron scattering measurements and γ-ray
spectroscopy following neutron-induced reactions. The precision of
these measurements is possible through well-tested experimental
setups and data analysis techniques. C6D6 scintillation detectors,
known for their timing and ability to use pulse shape
discrimination to differentiate between γ and neutron interactions,
are often used in precise neutron scattering measurements. The
electronics and data acquisition systems are monitored to ensure
there is no sporadic behavior in the detectors or their corresponding
electronics. While this method catches some abnormalities, there is
currently no method at UKAL to evaluate systematically large
sporadic or subtle long-term changes in the detector bias. The goal
of this research was to analyze the nominal energy of neutrons in
the recoil spectra of various elements at various angles and the
Compton edge of γ-rays in standard radioactive sources to see if
there was an observed shift in the detector bias. This evaluation
was completed by developing a PYTHON program that evaluated
all energy spectra.
The primary focus of the program was to find the channel number
associated with nominal neutron energy in each spectrum; and example is
shown by the blue dot in Fig. 2. This value is found by modeling a third
order polynomial to the curve and taking the absolute minimum of the
derivative. The shaded regions are the uncertainties in the measurements
and are found by taking the determinate of the covariance matrix produced
by the modeling function. These nominal energy channels were compared
to the behavior of their respective backups to see whether or not these
values exhibited shifts. Significant horizontal shifts are indicators of
changes in the detector bias and may result an increase or decrease in
counts for a given number of incident neutrons on the scattering sample.
An example of the channel number for a particular incident neutron energy
for the backup files (accumulated data saved every 30 minutes) and the
primary master file (the final cumulative data file for a given run) is shown
in Fig. 3. The program was written in PYTHON utilizing multiple
directories including NUMPY,SCIPY, and MATPLOTLIB.
The Program
Figure 1: The apparatus holding the detector whose energy spectra were analyzed by the
developed Python program. The blue drum holds the detector which is shielded with
polyethylene to minimize background radiation during the experiment.
Acknowledgements
This research was sponsored by the National Nuclear Security
Administration through NNSA/SSAP Grant # DE-NA0002931 and
the Cowan Physical Sciences Institute
The detector apparatus used at UKAL for the experimental
measurements is shown in Fig. 1. Neutrons scatter into the
detector and impart a portion of their energy to the scintillation
material resulting in the emission of light. This light produces an
electron at the cathode that is amplified in the photomultiplier
tube. The amplitude of the resulting output pulse is proportional
to the energy deposited in the detector. These signals are
digitized and after a statistically significant amount of counts are
collected, the resulting pulse-height spectrum will show a distinct
curve where the half height correlates to the maximum energy
the neutron can impart into the detector [see Figure 2] . This
curve was monitored over the course of the experiment to see if
the detector bias changes as a function of time of day or over the
course of the experiment.
Figure 2: A pulse-height spectrum for neutron scattering from12C at 20 degrees. The channel for the
Figure 5: The channel number corresponding to the nominal neutron energy at each an-
gle. The convergence of data near 80 degrees is unexpected and needs further investiga-
tion